As a technique for transmitting data using an audio signal or sound (sound wave) as a transmission medium, techniques described Patent Literatures 1 and 2 are known. In the technique described in Patent Literature 1, a modulation apparatus on a sound emission side modulates a spread code with a data code, performs differential encoding for the modulated spread code, multiplies by a carrier signal, performs a frequency shift, and outputs the differential code as a modulated signal. A demodulation apparatus on a sound collection side applies delay detection to an input modulated signal with a delay time for one chip of the differential code, detects synchronization between the delay-detected signal waveform and the spread code, and demodulates the data code based on the peak polarity of the detected synchronization point. In the technique described in Patent Literature 2, an electronic watermark is embedded in an audio signal by means of amplitude modulation, and the electronic watermark is extracted from the audio signal based on temporal and intensity features of fluctuation in amplitude.